Ductile fracture by void nucleation, growth and coalescence
Plastic deformation in ductile metals is limited by a mechanism in which voids, nucleated at second phase particles, grow and coalesce to form a crack. The results of a finite element solution for spherical elastic inclusions in a plastically deforming matrix are discussed. These results have been used in conjunction with experimental work using notched tensile specimens to generate multiaxial states of stress from which the local conditions leading to decohesion of the inclusion/matrix interface were determined. An important feature of these results is the statistical distribution of the interfacial strength. This distribution is bimodal, showing the presence of both weakly and strongly bonded particles. The latter have a modal strength of about 7 times the initial yield stress and the weakly bonded particles are assumed to be pre-existent. Experiments in plane and axisymmetric states of strain indicate that while the stress state is of relevance, the remote strain state is not. The absence of a macroscopic strain state effect is explained in terms of the statistical distribution of the voids nucleated from the population of randomly distributed inclusions. The stress and strain concentrations possible in local patches of high porosity have been investigated by a finite element approach based on the mechanics of a dilating continuum to determine void growth in the porous aggregate and the local conditions at failure. This investigation recognises the importance of the local hardening rate of the aggregate material and leads naturally to the idea of a size scale for failure, in the light of which the concept of a crack-like defect is re-examined.